Remove LintDiagnostic::msg

* instead simply set the primary message inside the lint decorator functions
* it used to be this way before [#]101986 which introduced `msg` to prevent
  good path delayed bugs (which no longer exist) from firing under certain
  circumstances when lints were suppressed / silenced
* this is no longer necessary for various reasons I presume
* it shaves off complexity and makes further changes easier to implement

compiler/rustc_errors/src/diagnostic.rs

@@ -200,8 +200,6 @@ pub trait SubdiagMessageOp<G: EmissionGuarantee> =
 pub trait LintDiagnostic<'a, G: EmissionGuarantee> {
     /// Decorate and emit a lint.
     fn decorate_lint<'b>(self, diag: &'b mut Diag<'a, G>);
-
-    fn msg(&self) -> DiagMessage;
 }
 
 #[derive(Clone, Debug, Encodable, Decodable)]